>>107315892I think he doesn't want anyone to talk about his past or remind him of it. Seeing his face invites questions about what could've caused his injuries, and that topic opens up old wounds (Padme's death, Obi-Wan's supposed betrayal).
Also, I think the helmet helps him stay detached from the world and hide from everything that could hurt him. If you see his face, he looks like a miserable dying wreck. But he's an angry guy, and I don't think he wants anyone to approach him, or feel pity/sympathy for him. The helmet makes sure people can't see his face or expressions, so they'll be uncertain and intimidated in his company. This helps him keep everyone away.
However, he doesn't seem to react with violence every single time when someone sees his face. ESB had that scene where Piett saw the back of his head, but neither man reacted.
Maybe Vader's okay with people who know to keep their mouth shut about uncomfortable things. Piett was the guy who stood right there while Ozzel was strangled, and he also somehow got spared after the Millennium Falcon escaped. Both times he reacted with silence and tried very hard to keep his emotions contained in a terrifying situation. So I think Vader prefers stoicism over pleas, apologies or excuses; they only serve to remind him of stuff that pisses him off.
